This moves the `sources_tests` into ./test/data/sources/ and makes the
entire test use `locate_test_data()` to get relative paths for their
accesses.
This further improves our test cases to support running from
installments rather than local checkouts. We need access to ./test/data
guarded, so we can install packages and still have the tests access the
correct paths.
This also adjusts the HTTP-Server we use in the test to serve data
relative to a path it is handed. I now chose `./test/data`, which will
easily allow us to re-use the same HTTP-Server in the future for other
tests that require it.
The idea is that source can themselves spawn other modules, esp.
new secrets modules. For this they need to know the library dir,
aka 'libdir' throughout the osbuild source. Therefore change the
SourceServer to directly get the library directory instead of
just the sub-directory to the sources. Then pass the library
directory to via the JSON API to the source.
Adjust all usage of the SourceServer, including the tests.
